Re: [Boost-users] Python 'in' keyword-like in C++

From: Simon Buchan (simon_at_[hidden])
Date: 2005-09-25 18:49:22

David Hall wrote:
> Anything like "while(int i <for_all_in> vec) cout << i;" isn't all that 
> feasible (without being really hackish), since you need to maintain some 
> sort of state inside of something that lives beyond each while 
> statement, but specifically not in for_all_in, since we want to be able 
> to iterate over the same container twice.
> 
Well, I didn't really mean for that to be taken _too_ seriously, I just
was throwing out general concepts.
> The best I can come up with is this:  (This could be prettified if we 
> had auto, with the added bonus of allowing for autoselection of const 
> versus non-const iterators.)
>    for_all_in_iterator<std::vector<int> > i;
>    while( i <for_all_in> vec)          std::cout << *i << std::endl;
> 
> where for_all_in_iterator is essentially a specialized form of 
> boost::optional<>.  I don't think you can get around using an iterator, 
> unless you're willing to sacrifice either for_all_in's reentry or status 
> as pseudo-keyword.
> 
Gosh! You didn't need to do all that!
> Either way, I added my code in, with a few edits to yours, to allow for 
> non-const RhsT. Ideally, there'd be a for_all_in_const_iterator version, 
> but that's pretty easy.
> 
Yes, you need the overloads for rvalue arguments, to do things like 
'if(6 <is_in> foo) ...' I put that in after reading up on 'perfect 
forwarding' (although I used the equivilant '(T const & foo)' and did it 
for LhsT _and_ RhsT, just to 1-up you :D)